It is well known to form shaped articles from a thermoplastics material by extruding a tube of the thermoplastics material, repetitively enclosing a length forming the end of the extruded tube within a mould cavity, separating the tube lengths from the parent tube, and after separation inflating them individually with pressurised gas so as to conform them to the mould cavity and so form the shaped articles. Such a process in its basic form, usually referred to as "blowmoulding", is used, inter alia, for the manufacture of bottles for household and industrial use, in applications where the internal pressure is to be little or no greater than atmospheric pressure or, in other words, there is little or no over-pressure. Bottles for carbonated beverages, however, may be required to withstand over-pressures of several atmospheres, and for such bottles a modification of the basic blowmoulding process, usually referred to as "stretch-blowmoulding", is used. In this proces the individual lengths of extruded tube are stretched mechanically in their longitudinal direction prior to, or at the same time as, they are inflated; in a variation, injection-moulded preforms are used instead of the lengths of extruded tube. The longitudinal stretching together with the circumferential stretching which occurs during inflation molecularly orientate the bottle material both longitudinally and circumferentially of the bottles; this biaxial orientation gives the bottles the strength necessary for them to withstand the high over-pressures which may be generated by carbonated beverage products.
Another kind of container which may be subject to substantial over-pressures is the conventional three-piece food can, which is liable to over-pressure during processing by the food packer and/or during reheating by the consumer. Such cans are typically made of tinplate. This material is becoming expensive.